The present invention is a process for the carbothermic reduction of silicon dioxide to elemental silicon, where carbon levels in the smelting furnace are adjusted based on evolved carbon monoxide amounts.
Elemental silicon is produced by the carbothermic reduction of silicon dioxide (SiO.sub.2) according to the overall reaction: EQU SiO.sub.2 +2C.fwdarw.Si+2CO (1)
It is well known to those skilled in the art that this reaction proceeds through a number of intermediate reactions involving the production and reaction of silicon monoxide (SiO) and silicon carbide (SiC). Key intermediate reactions for the purpose of this invention can be summarized as: The reaction of silicon dioxide with carbon to form silicon monoxide, EQU 2SiO.sub.2 +2C.fwdarw.2SiO+2CO (2)
the reaction of silicon monoxide with carbon to form silicon carbide, EQU SiO+2C.fwdarw.SiC+CO; (3)
and the reaction of silicon carbide with silicon monoxide to form elemental silicon, EQU SiO+SiC.fwdarw.2Si+CO. (4)
The sum of the reactions described in equations (2) through (4) result in the overall reaction described in equation (1).
As equation (1) illustrates, theory would suggest that for a silicon smelting furnace to be in carbon balance two moles of carbon should be added per mole of silicon dioxide. This condition is described as 100 percent carbon theory. However, due to process inefficiencies, operation of a silicon smelting furnace does not proceed exclusively according to reaction (1).
If, for example there is insufficient or unreactive carbon, in bulk or locally, to effect reaction (3) a portion of the silicon monoxide will exit the charge bed in the offgas. This situation can occur due to raw materials selection, bed design, and unbalanced stoichiometry in the bed. The loss of silicon monoxide from the reactor results in reduced recovery of elemental silicon. In addition, in the case of insufficient carbon, increased consumption of carbon electrodes used in the furnace can occur. In extreme cases of carbon deficiency, carbon used as furnace lining may be consumed.
Conversely, if too much carbon is present in the furnace, in bulk or locally, reaction (3) can predominate causing silicon carbide accumulation and reduced silicon production. The accumulated silicon carbide can cause filling of the furnace causing the electrode to be raised out of the proper operating position. In addition, when excess silicon dioxide is added to react with the accumulated silicon carbide, the additional silicon monoxide formed can cause increased electrode consumption and loss of yield.
Therefore, it is important for efficient furnace operation that the furnace be kept in carbon balance. However, as a result of the described inefficiencies, carbon balance in a silicon smelting furnace cannot be maintained by merely adding carbon and silicon dioxide to the furnace in a two to one molar ratio, based on carbon theory.
The inventors describe a process herein for the carbothermic reduction of silicon dioxide to elemental silicon where the emitted amount of carbon monoxide in furnace offgas is measured and the ratio of carbon monoxide to the amount of silicon dioxide added to the furnace determined. Based upon this ratio the carbon balance of the furnace can be determined and corrected as needed.